Prediction Of Departure From Nucleate Boiling Critical Heat Flux In A Round Tube Based On CFD Simulation

Nucleate boiling enables high heat transfer capacity which is desired in pressurized water reactor(PWR)core.But when the wall heat flux exceeds a critical value,bubbles crowd near the heating surface which detoriates the heat transfer ability,which consequently leads to the occurrence of departure from nucleate boiling(DNB)and wall temperature excursion.The DNB can cause damage or melting of fuel rods.Hence,the prediction of CHF of DNB type is significant in thermal-hydraulic analysis in PWR.Traditionally,empirical correlations and/or the CHF look-up table based on large amount of experimental data are utilized for CHF prediction.As a consequence of development of two-phase computational fluid dynamics(CFD)technology,numerical prediction of CHF becomes potentially an important option.In this dissertation commercial CFD software STAR-CCM+ is utilized to investigate the approach for numerical prediction of DNB-type CHF.Since the CFD prediction of DNB-type CHF is based on the distribution of void fraction,especially near the wall,the DEBORA experiments which include detailed void fraction distribution data at the outlet is utilized to investigate the performance of two-phase CFD in the prediction of nucleate boiling heat transfer in the upward flow in a uniformly heated round tube.Detailed sensitivity analysis has been carried out to figure out the model or parameter which have significant influence on the radial distribution of void fraction,gas velocity,liquid temperature and bubble diameter,and axial distribution of wall temperature.The interphase models are calibrated by adjusting crucial model parameters based on one selected set of experimental data.The influence of the droplet diameter model,i.e.Kurul-Podowski and S-Gamma models is also investigated.Finally,the calibrated model is applied to other working conditions to verify its applicability,and reasonable agreement has been achieved.On the basis of model calibration,CFD simulation is applied to predict CHF based on DNB criterion from Weisman.The calculation results are compared with the corresponding test data to evaluate the ability to predict based on this criterion and the distinct effects between the two bubble size models.